1. Field of the Invention
The present invention relates generally to alkaline nickel-zinc cells and their preparation, and specifically to the formation/activation procedures for nickel-zinc cells.
2. Background Art
Rechargeable cells such as, for example, Nickel/Zinc, Silver/Zinc, Zinc/Air and Manganese-dioxide/Zinc, containing zinc electrodes are of significant interest due to the abundance and, therefore, low cost of zinc, as well as low equivalent weight, high Coulombic efficiency, reversible electrochemical behavior, and reduced environmental disposal problems (e.g., in comparison to lead or cadmium).
Before use as an individual cell or as a group of cells in a battery pack, Nickel-Zinc cell/s needs to be formed. Many inventions have identified different procedures with the hope of forming the Nickel-Zinc cell/s with uniform characteristics (for example initial discharge capacity). For example, Pyun, et al. (U.S. Pat. No. 5,563,008 dated Oct. 8, 1996) describes a procedure in which a nickel electrode is first charged and discharged outside the cell before assembly in to the actual cell. The procedure obviously involves an extra external processing step, requiring additional cells to carry out the procedure. The electrode needs to be washed and dried before assembling into the actual cell, which leads to an increase in the assembly time and cost.
Terasaka, et al. (U.S. Pat. No. 5,405,714 dated Apr. 11, 1995) describes a method to form Nixe2x80x94Zn cells using non-sintered type Nickel positive electrodes, in which the cells are rested at room temperature until their potential drops to the equilibrium potential of Co/Co(OH)2 (about 0.50-0.60 volts). However, at this potential, Co is soluble in KOH, and the resulting soluble species migrates over to the zinc electrodexe2x80x94leading to deposition of Co on the zinc electrode. Deposition of Co on Zn forms a Co/Zn micro-cell that leads to discharge of the zinc electrode and formation of hydrogen gas. This step is also a time consuming step as the cell potential has to reach the rest potential on its own.
Thus, there is a need for a formation procedure that can be applied to a complete assembled cell as well as a procedure that is applicable to Nickel-Zinc cells made with non-sintered and sintered nickel cathodes.
The present invention comprises a formation procedure/process for alkaline nickel-zinc cells, the procedure comprising the steps of a) assembling an alkaline nickel-zinc cell, b) charging the cell at a charging current rate until a first condition is achieved, wherein at the first condition the cell has a first cell voltage, c) maintaining the cell at the first cell voltage while applying a maintenance current to the cell until a second condition is achieved, and d) discharging the cell at a discharging current rate until a third condition is achieved, wherein the steps of charging, maintaining and discharging are repeated at least one time, and, thereafter e) the cell is finally charged at a final charging rate until the final condition is achieved. Through this process, the nickel-zinc cell can have improved cycle life, and more uniform cell properties, which make them suitable for use in a battery pack.
In the current procedure, the charging current rate is between approximately 0.05C and approximately 0.2C, and preferably the charging current rate is approximately 0.1C. The charging current is applied to the cell until the first condition is achieved, wherein the first condition can comprise either the passage of a period of time or a specific cell voltage. If the first condition comprises the passage of a period of time, it preferably comprises a period of time between approximately 7.5 hours and 30 hours, and even more preferred the period of time comprises approximately 15 hours. On the other hand, if the first condition comprises a cell voltage, it preferably comprises a cell voltage of between approximately 1.95 Volts/cell and 2.05 Volts/cell, and even more preferably comprises a cell voltage of approximately 2.02 Volts/cell.
After the charging step is the maintaining step. In this step, the cell is maintained at a specific voltage until the second condition is met. Preferably, the second condition comprises maintaining the cell at the specific cell voltage for a period of time between 10 minutes and 30 minutes, and even more preferably a period of time of approximately 20 minutes. Alternatively, the second condition comprises maintaining the cell at the specific cell voltage until the maintenance current rate drops to between approximately 0.01C and approximately 0.03C, and even more preferably to 0.02C.
The discharging step follows the maintaining step. In this step, the cell is discharged at a constant discharge current rate until a third condition is achieved. Preferably, the discharge current rate is between approximately 0.4C and approximately 0.6C, and even more preferably comprises a current rate of 0.5C. The third condition comprises the passage of a period of time, preferably a period of time of between approximately 2 and 3 hours, and even more preferably the period of time comprises approximately 2.5 hours.
Once the steps of charging, maintaining and discharging are completed, it is preferred that those steps are repeated between 2 and 4 times, and more preferably that those steps are repeated 3 times.
After the steps have been repeated, the step of final charging occurs. The final charging step requires the application of a final charging current rate for a period of time. Preferably, the final charging rate is between approximately 0.05 and 0.2C, and even more preferably is approximately 0.1C. This current is applied until the final condition is achieved. The final condition may be achieved either by the passage of a period of time or by a specific cell voltage. If the final condition comprises the passage of time, the period of time preferably comprises between approximately 7.5 hours and 30 hours, and even more preferably comprises 15 hours. If the specific cell voltage is used, the final condition preferably comprises a cell voltage of between approximately 1.95 Volts/cell and 2.05 Volts/cell, and even more preferably comprises approximately 2.02 Volts/cell.